138 PRACTICAL ASTRONOMY 



measure triangles. When this need occurs, each angle of the 

 triangle should be measured directly. If but two angles are 

 measured and their sum is subtracted from 180 to get the 

 third, all errors of measurement of the two angles are thrown 

 into the third angle. When all the angles are measured to a 

 high degree of precision, their sum will ordinarily be more or 

 less than 180, indicating an impossible triangle. To make the 

 triangle possible, the angles are adjusted so that their sum 

 shall be 180. The adjustment is effected by dividing the total 

 error equally among the three angles. It might seem that a 

 distribution in some ratio to the size of the angles should be 

 adopted; but the method applied considers that there is no 

 more reason for making an error in measuring a large angle 

 than in measuring a small angle, which is probably true. 



PRACTICAL ASTRONOMY 



DEFINITIONS AND TERMS 



LATITUDE AND LONGITUDE. 



If a meridian, that is, a circle passing through the axis of the 

 earth, be passed at a given point of the earth's surface, the 

 angular distance of the point from the equator, measured on 

 the meridian, is the latitude of that point. A plane parallel to the 

 equator cuts the earth's surface in a circle called a parallel of 

 latitude. All the points on a parallel of latitude have the same 

 latitude. The longitude of a place is the angle that the plane 

 of the meridian of the place makes with the plane of a refer- 

 ence meridian (usually the meridian of Greenwich). This 

 angle may be measured on the equatorial circle or on the 

 parallel of latitude of the given place. Longitude is counted 

 from the reference meridian toward the west. 



THE CELESTIAL SPHERE 



The celestial sphere is an imaginary sphere enclosing all the 

 heavenly bodies. It is of such enormous dimensions that, 



